Effects of Supercritical CO₂ Immersion Time on CO₂/CH₄ Gas Seepage Characteristics in Coal

Abstract

Low permeability has always limited the efficient extraction of coalbed methane (CBM) in China. To investigate the permeability enhancement effect of supercritical CO₂ on coal seams, experiments were conducted using a self-developed supercritical CO₂ immersion system and a single-component gas (CO₂ and CH₄) seepage experimental apparatus, considering different immersion times and injection pressures. The gas seepage characteristics of CO₂ and CH₄ in coal seams were studied under various conditions. Additionally, nuclear magnetic resonance (NMR) was used to obtain the porosity components of the coal samples at different immersion times. The changes in permeability before and after the experiment were compared to analyze the permeability enhancement effect of supercritical CO₂ on the coal samples. The results show that the original porosity of the coal sample was 2.06%. After 5, 10, 15, and 20 days of immersion, the porosity of the coal samples increased by 2.78%, 3.26%, 3.22%, and 2.86%, respectively. After immersion in supercritical CO₂, the porosity exhibited a trend of initially increasing and then decreasing. During the single-component gas seepage experiment following supercritical CO₂ immersion, the outlet flow rates of both CO₂ and CH₄ reached their maximum on the 10th day of immersion. Compared with the 0-day immersion, the outlet flow rates of CO₂ and CH₄ increased by 4.49 times and 3.23 times, respectively. After immersion, the CH₄ permeability within the coal sample was stronger than that of CO₂.

1. Introduction

Coal is one of the main energy sources in our country, and China is also one of the world’s most coal-rich nations [1,2,3]. Mine gas (coalbed methane) disasters are one of the major hazards in coal mines [4,5]. Due to the complex structure of coal seams, high degree of metamorphism, soft coal quality, and low permeability in our country, pre-mining gas extraction from original coal seams is difficult [6,7]. Efficient reservoir modification technologies are urgently needed for economically viable development [8]. Currently, methods such as intensive drilling, hydraulic fracturing, hydraulic jetting, hydraulic slitting, and gas injection fracturing are used to improve the recovery rate of coalbed methane [9,10]. Although certain achievements have been made, these technologies still exhibit inherent limitations. For instance, the intensive borehole drainage technique suffers from issues such as excessive drilling workload, borehole collapse or clogging, and poor geological adaptability. Conventional large-scale hydraulic fracturing, a key method for enhancing coalbed methane production, is constrained by its high water consumption, potential reservoir damage, water-blocking effects, and risks of groundwater contamination [11]. More critically, the hydraulic fracturing process may induce seismic events, exacerbating environmental and safety concerns.

In recent years, waterless fracturing technologies (e.g., CO₂/liquid N₂ gas-phase fracturing) have emerged as a promising direction for unconventional gas extraction worldwide. To address the challenges of low-permeability coal seam gas drainage, researchers have proposed injecting CO₂ into coal seams. This approach leverages the higher adsorption affinity of coal for CO₂ compared with methane, thereby displacing the adsorbed methane and enhancing coalbed methane recovery [12]. However, while this method demonstrates short-term permeability improvement, it is primarily suitable for localized gas control in underground coal mines rather than large-scale field applications. To address the aforementioned challenges, supercritical CO₂ fracturing technology has emerged. Supercritical CO₂ refers to a fluid with a temperature greater than 31.1 °C and pressure exceeding 7.38 MPa [13]. This fluid not only possesses the high-density characteristics of a liquid but also has the low-viscosity characteristics of a liquid [13]. Additionally, it exhibits exceptional flow and permeability properties, as well as the ability to extract small molecular compounds from coal. Injecting supercritical CO₂ into coal seams can, on the one hand, further promote the effective development of the coal seam fracture system during coalbed methane extraction, increasing the flow channels for coalbed methane. On the other hand, it can reduce the partial pressure and enhance the desorption rate of coalbed methane from the coal matrix. As a result, there is an increasing demand for injecting supercritical CO₂ into low-permeability coal seams for coalbed methane extraction, offering promising prospects for application and development.

Y Yu and others [14] conducted experimental studies on the flow, permeability, and adsorption of supercritical CO₂ in coal seams, considering the influence of the injection pressure and temperature. They compared the changes in the longitudinal wave velocity of the samples before and after the experiments. Jiang R and others [15] performed geochemical interaction experiments with typical high-ash coal and CO₂, investigating the changes in water-soluble elements after the interaction with supercritical CO₂. Niu Q and colleagues [16] injected supercritical CO₂ into coal samples of different ranks for permeability testing, and their results showed that the permeability of coal is related to its rank. In particular, medium-rank coals, which have more fractures generated during coalification, showed the least effect on permeability after cyclic loading/unloading. Hashemi S S [17] studied the impact of supercritical CO₂ on the fracture permeability and surface characteristics of shale with different compositions. Li W and others [18] conducted a series of studies on naturally fractured coal, coal adsorbing supercritical CO₂ for different periods, and a high-pressure triaxial apparatus with supercritical CO₂. They found that the coal treated with supercritical CO₂ exhibited a well-connected pore system, with an increased proportion of mesopores and macropores, as well as an overall increase in pore volume.

In summary, current research both domestically and internationally is largely focused on the geochemical reactions of supercritical CO₂ with coal and its impact on the coal’s permeability. These studies have demonstrated the potential of supercritical CO₂ in enhancing coal seam permeability and improving gas extraction efficiency. The findings highlight how CO₂ affects the structural characteristics of coal, such as the pore distribution and connectivity, thereby increasing the recovery of coalbed methane in low-permeability coal seams. However, there is still limited research on the permeability changes following the dissolution of coal with supercritical CO₂. This study focused on the 8# coal of the Benxi Formation in the Ordos Basin and conducted supercritical CO₂ solvent immersion treatments on coal pillar samples for different durations. Permeability experiments on coal pillar samples were carried out to investigate the relationship between the treatment duration, gas seepage pressure gradient, and permeability parameters. In conjunction with NMR technology, the impact of the immersion time on the coal body porosity was analyzed. The findings of this study contribute to enriching the theoretical foundation for enhancing coalbed methane recovery and geological storage using supercritical CO₂.

2. Experimental Methods

2.1. Subsection

2.1.1. Coal Sample Selection and Pretreatment

In this experiment, coal samples from the Benxi Formation 8# coal seam of the Upper Paleozoic coal-bearing strata in the Ordos Basin were selected. The raw coal was immediately sealed in fresh-keeping film after being extracted from the mine to prevent damage during transportation and to ensure the coal sample maintained its original structure. Using a core-drilling machine, the raw coal was processed along the vertical bedding direction into cylindrical specimens with a diameter of 10 mm and a height of 20 mm. Only coal samples with smooth surfaces and no obvious cracks were selected for the dissolution and permeability experiment. The coal composition information is shown in

Table 1. Results of industrial analysis of coal sample.

Designation	Industrial Analysis/%
	(Mad)	(Vad)	(Aad)	(FCad)
Benxi Group 8# coal sample	0.74	12.64	26.00	60.62

Note: M_ad denotes moisture, V_ad denotes volatile matter, A_ad denotes ash, and FC_ad denotes fixed carbon.

.

To study the effect of supercritical CO₂ dissolution on the permeability characteristics of coal before and after treatment, the coal samples were first subjected to a drying pre-treatment in a drying oven at 70 °C. The coal samples were weighed every 3 h, and if the weight change was less than 0.01 g or the weight remained unchanged for two consecutive measurements, the samples were considered to be fully dried. Next, 200-mesh fine sandpaper was used to grind and polish the coal surface to remove any impurities. Finally, the prepared coal samples were placed into the dissolution chamber for the supercritical CO₂ dissolution experiment. After a certain period of dissolution, the samples were subjected to CO₂ permeability experiments to evaluate the changes in their permeability.

2.1.2. Experimental Program

(1)

Coal samples in supercritical CO₂ medium solution leaching treatment

The treatment of coal samples with supercritical CO₂ dissolution was conducted using a supercritical CO₂ dissolution experimental system, as shown in Figure 1.

This system consisted of a CO₂ gas cylinder (40 L), a CO₂ storage tank with a viewing window (with a maximum withstand pressure of 30 MPa and a volume of 2.2 L), a high-temperature circulator (0–90 °C, with a temperature control accuracy of ±0.5 °C), a heating and cooling circulator (−40 °C–80 °C, with a temperature control accuracy of ±0.3 °C), pressure sensors (0–20 MPa; with an accuracy of 0.2% FS), temperature sensors (−50 °C–200 °C, with an accuracy of ±0.2 °C), a vacuum pump (1400 r/min; 220 v), a pressure-regulating valve (R21-1/4-4P), a safety valve (DN10-25 mm), and a data acquisition system (64 channels), among other components. During the experiment, supercritical CO₂ was supplied from the gas cylinder through a booster pump and heating–cooling circulator, which worked together to pressurize and heat the CO₂. An online intelligent temperature controller (with an accuracy of 0.1 grade and a display size of 7 inches) was used to collect pressure and temperature signals throughout the experimental process.

The specific method for dissolving the coal samples in the supercritical CO₂ medium was as follows: The dried coal samples were placed into a high-pressure dissolution storage tank. Supercritical CO₂ was then injected into the tank until the coal samples were fully submerged. After the set soaking time, the coal samples were removed from the tank and placed into vacuum bags for sealed storage. The dissolution procedure is outlined in

Table 2. Experimental program of supercritical-state CO₂ dissolution leaching of coal body.

Serial Number	Drying Time (h)	Temperature (°C)	Pressure (MPa)	Dissolution Time (d)	Note
1	8.0	65.0	8.0	Raw coal sample	Insoluble leaching treatment Supercritical CO2 dissolution leaching treatment
2	8.0	65.0	8.0	5.0
3	8.0	65.0	8.0	10.0
4	8.0	65.0	8.0	15.0
5	8.0	65.0	8.0	20.0

.

(2)

Permeability experiment of coal samples after immersion under single-component gas (CO₂/CH₄) conditions.

This experiment used a single-component CO₂ and CH₄ percolation experimental setup, as shown in Figure 2.

The setup consisted of six main units: the gas supply unit, gas pressurization unit, confining pressure loading unit, clamp, back-pressure unit, and data acquisition unit.

The gas supply unit was composed of a CO₂ gas cylinder (40 L).

The gas pressurization unit consisted of an air compressor, booster pump, and pressure-regulating valve.

The confining pressure loading unit was composed of a self-developed hand-operated pump device.

The back-pressure unit was composed of a self-developed back-pressure valve.

The data acquisition unit included a Coriolis mass flowmeter, flow sensors, and a 64-channel data acquisition system.

The percolation experimental method for the coal samples after impregnation under single-component CO₂ and CH₄ gas conditions was as follows: The cylindrical coal samples, before and after supercritical CO₂ impregnation, were placed into the test vessel. Firstly, a CO₂ percolation experiment was conducted using the single-component gas percolation experimental setup. Pressure- and flow-monitoring devices were installed to collect real-time pressure and flow data at both the gas injection and outlet points. After each experiment, the coal column was replaced, and CH₄ gas was injected. The pressure and gas flow changes during the percolation process were recorded in real time. The tests were repeated for coal samples with different impregnation times. The tested coal samples are shown in Figure 3, and the CO₂ and CH₄ gas percolation experimental scheme is shown in

Table 3. Experimental program for one-component gas seepage.

Name of Experiment	Seepage Gas	Injection Pressure (MPa)	Name of Experiment	Seepage Gas	Injection Pressure (MPa)
CO2 seepage experiment	CO2	1.0	CH4 seepage experiment	CH4	1.0
		2.0			2.0
		3.0			3.0
		4.0			4.0
		5.0			5.0
		6.0			6.0
		7.0			7.0

.

During the gas injection process, the gas compressibility factor changed with the increase in the gas injection pressure, making it difficult to describe the gas migration process in the coal column using a fixed volumetric flow rate. Therefore, the average outlet flow rate was introduced to solve this problem. Assuming the average pore pressure was the average value of the coal column injection pressure and outlet pressure, according to Boyle’s law, we can obtain Equation (1) [19].

$${\displaystyle \frac{\left(P_1+P_2\right)}{2}}\overline{Q}=P_2{Q}_2$$

(1)

In Equation (1), P₁ and P₂ represent the inlet and outlet pressures at both ends of the coal column (MPa); the average outlet gas flow rate is represented as $\overline{Q}$ (mL); and the outlet gas flow rate is represented as Q₂ (mL).

During the gas injection process, it can be assumed that the average outlet gas flow rate is linearly related to the pressure gradient. Therefore, it is assumed that the gas percolation process within the coal column follows Darcy’s law. By substituting Equation (1) into Darcy’s law, we can obtain Equation (2) [20].

$$k={\displaystyle \frac{\mu L\overline{Q}}{\left(P_1-P_2\right)A}}={\displaystyle \frac{2\mu Q{P}_{\alpha }L}{\left(P_1^2-P_2^2\right)A}}$$

(2)

In Equation (2), k is the permeability of the experimental fluid (10–15 m²); μ is the dynamic viscosity of the experimental fluid (μ Pa·s); Q is the fluid flow rate (ml); Pα is the standard atmospheric pressure (0.1 MPa); L is the length of the coal column specimen (cm); and A is the cross-sectional area of the specimen (cm²).

2.2. Experimental Preparation Stage

The experimental steps are shown in Figure 4.

(1)

The coal samples were placed in a high-pressure leaching vessel, where supercritical CO₂ was injected for the leaching experiments at different time intervals. After leaching, the coal samples were placed in a vacuum-pressurized water saturation device, where they were evacuated to saturation. The pore structure of the water-saturated coal samples and the pore structure of the centrifuged coal samples were measured using NMR systems and a centrifuge.

(2)

Based on the pressure distribution characteristics of the coal reservoir in the study area, the pressure in the experimental conditions was adjusted according to the corresponding strength of the sample using similarity theory. Given that the experimental coal samples were relatively small with lower strength, the confining pressure and gas pressure were proportionally reduced to reflect the field conditions. The coal columns, leached for different time intervals, were placed into a loading device, and the confining pressure was gradually applied up to 8 MPa.

(3)

Before the experiment began, a vacuum pump was used to evacuate the coal column inside the loading device. The data acquisition unit was then activated, and the gas inlet valve was opened. Under a constant temperature of 300 K, pure CO₂ gas was injected into the loading device at a constant pressure ranging from 1 to 7 MPa. By adjusting the pressure-regulating valve, the injection pressure was gradually increased from 1 MPa to 7 MPa. Simultaneously, the flow rates at the gas inlet and outlet were measured. Once the flow rates stabilized, the injection pressure was increased in steps, and the flow values at different pressure gradients were recorded.

(4)

At the end of the experiment, the data acquisition system and the gas cylinder valve were first closed. The pressure relief valve was then opened under ventilated conditions to release the gas pressure from the inlet-end pipeline. Next, the back-pressure valve was adjusted to 0, and the gas pressure accumulated at the outlet was released. Finally, the confining pressure was removed, and the coal column was taken out by opening both ends of the loading device.

(5)

The coal samples were replaced with those leached for different durations, and the data were recorded. Steps (3) to (4) were repeated.

(6)

The CO₂ gas was replaced with CH4 gas, and steps (3) to (4) were repeated.

3. Results

3.1. Pore Distribution in T₂ Mapping of Water-Saturated Coal Bodies for Different Dissolution Leaching Time Lengths

According to B.B. Hodorot’s classification method [21], the T₂ NMR spectrum area reflects the pore volume, and the connectivity or isolation between peaks indicates the pore connectivity. T₂ < 10 ms corresponds to micropores, 10 ms ≤ T₂ ≤ 100 ms corresponds to mesopores, and T₂ > 100 ms corresponds to macropores and microfractures [21]. The T₂ NMR spectrum distribution of the experimental coal sample is shown in Figure 5.

Under fully saturated conditions, the T₂ curve of the coal sample exhibits two peaks in the ranges of 0.01–10 ms and 10–1000 ms. The spectrum of the coal samples shows pores of varying sizes. An increase in the width of the T₂ peak range indicates an expansion of the pore size, while an increase in the amplitude of the T₂ peak signals indicates an increase in the number of pores [22]. Both factors suggest that after supercritical CO₂ dissolution, the microfracture network of the coal sample was more developed.

In Figure 5, it can be seen that after immersion in supercritical CO₂ for different durations, the T₂ spectrum of the fully saturated coal body shows an increase in both the peak width and signal amplitude in comparison with the original coal sample before immersion. At the initial immersion time of 5 d (Figure 5b), the pore structure of the coal sample underwent damage, as reflected in the T₂ spectrum by the increase in the amplitude and area of micropores and meso/macropores. When the immersion time was extended to 10 d (Figure 5c), the damage to the coal structure gradually accumulated, and the degree of pore structure destruction intensified. This is reflected in the T₂ spectrum by the largest number of micropores and meso/macropores. At immersion durations of 15 d–20 d (Figure 5d,e), the number of pores decreased to some extent, but the degree of pore destruction remained much higher than that of the original coal (Figure 5a). This suggests that when the immersion time exceeded 10 d, further increases in the immersion duration did not effectively enhance the number of pores or significantly improve the pore space of the coal body.

3.2. Changes in Porosity Fractions and Cumulative Porosity of Water-Saturated and Centrifuged Coal for Different Dissolution Leaching Durations

Figure 6 shows the changes in the porosity components and cumulative porosity of saturated and centrifuged coal after supercritical CO₂ impregnation for different durations. As seen in Figure 6, with an increase in the supercritical CO₂ impregnation time, the amplitude of the porosity components and cumulative porosity of the saturated coal first increased in the 5 d–10 d range and then decreased in the 10 d–20 d range. After the supercritical CO₂ treatment, not only did the porosity amplitude increase, but the pore size distribution also became broader. The proportion of micropores and mesopores in the coal sample was significantly higher than that of macropores. The original coal sample had a porosity of 2.06%, but after 5 d, 10 d, 15 d, and 20 d, the porosity increased by 2.78%, 3.26%, 3.22%, and 2.86%, respectively. The porosity of the coal sample after supercritical CO₂ impregnation showed a trend of first increasing and then decreasing.

This occurred because, in the presence of water, supercritical CO₂ formed weakly acidic carbonic acid, which then rapidly decomposed to produce H⁺. These ions could interact with the mineral components of the coal sample, initiating geochemical reactions. The reaction characteristics of the minerals in coal are related to their chemical properties. Under acidic conditions, the carbonate minerals in coal react more intensively, leading to the dissolution of the associated elements. During the reaction process, minerals may be partially or entirely leached. For example, calcite can undergo dissolution in a carbonic acid solution, with the reaction equation given in Equations (3)–(5) [23]. This process also promotes the detachment of clay minerals, as shown in Equation (6) [23]. The leached minerals create new pores where the original minerals were located, thereby facilitating the development of micropores. This can lead to the dissolution of aluminosilicate minerals, such as feldspar, and the formation of secondary kaolinite precipitates, as described by the reactions in Equations (7) and (8) [23]. However, as the reaction progressed and the reaction time exceeded 10 days, a large number of bicarbonate ions in the water dissociated, reforming carbonate ions, which, in turn, led to the reformation of calcite crystals. The continuous accumulation of these crystals and precipitates may have clogged the previously opened channels, leading to a gradual decrease in the porosity of the coal body when the leaching times were 15 d and 20 d.

$$C{O}_2+H_{2}O\to H_{2}C{O}_3$$

(3)

$$H_{2}C{O}_3\to H^{+}+HC{O_3}^{-}$$

(4)

$$CaC{O}_3(Calcite)+H^{+}\to C{a}^{2+}+HC{O_3}^{-}$$

(5)

$$A{l}_{2}S{i}_2{O}_5{(OH)}_4(Kaolin)+6H^{+}\to 2A{l}^{3+}+2H_{4}Si{O}_4+H_{2}O$$

(6)

$$KAlS{i}_3{O}_8(Potassium-feldspar)+2C{O}_2+11H_{2}O\to A{l}_{2}S{i}_2{O}_5{(OH)}_4(Kaolin)+2K^{+}+2HC{O_3}^{-}+4H_{4}Si{O}_4$$

(7)

$$NaAlS{i}_3{O}_8(Soda-feldspar)+2C{O}_2+11H_{2}O\to A{l}_{2}S{i}_2{O}_5{(OH)}_4(Kaolin)+2N{a}^{+}+2HC{O_3}^{-}+4H_{4}Si{O}_4$$

(8)

This was because after the coal came into contact with supercritical CO₂, CO₂ molecules entered the primary pores of the coal, promoting the development of primary fissures. Meanwhile, the entry of CO₂ molecules created more new secondary pores and cracks, which may be influenced by gas migration, further increasing the porosity. Additionally, supercritical CO₂ dissolved and extracted certain inorganic and organic minerals from the coal, enhancing the connectivity of the pores within the coal body. Pores that were blocked by mineral components were also connected, allowing for a more developed porosity and an overall increase in the coal sample’s porosity.

3.3. Time-Varying Characteristics of Gas Seepage in Coal Samples with Different Leaching Times

Figure 7 and Figure 8 show the gas flow-time curves of the single-component CO₂ and CH₄ for coal samples with different impregnation durations. As seen in the figures, CO₂ and CH₄ exhibited different flow characteristics in the same permeable coal. Both gases increased in their flow rate with the rising injection pressure, and the outlet flow rate showed a stepwise increase over time, while the outlet pressure remained almost unchanged. Additionally, under the same injection pressure, the outlet flow rate of CO₂ exhibited a trend of first increasing and then decreasing as the coal impregnation time increased.

For the impregnation times of the raw coal sample, 5 d, 10 d, 15 d, and 20 d, the maximum outlet CO₂ flow rates were approximately 1674 mL/min, 2455 mL/min, 7525 mL/min, 4996 mL/min, and 3486 mL/min, respectively. The maximum outlet CH₄ flow rates were approximately 2730 mL/min, 3538 mL/min, 8831 mL/min, 6762 mL/min, and 5479 mL/min, respectively. Both CO₂ and CH₄ exhibited the highest outlet flow at 10 d. Compared with the 0 d impregnation, the outlet flow of CO₂ and CH₄ at 10 d increased by 4.49 times and 3.23 times, respectively.

This indicates that during the 5–10 d impregnation period, the pore structure of the coal changed, increasing the number of mesopores and macropores used for seepage. This led to an increase in the seepage channels, causing a macro-level increase in the outlet flow. However, during the 15 d–20 d period, the outlet flow of CO₂ and CH₄ decreased. This is because, during this period, supercritical CO₂ dissolved the minerals in the coal. After mineral dissolution, the minerals deposited in larger pores, and the pores where minerals were dissolved became narrower. This narrowing of the pore openings resulted in a reduction in the effective seepage area, leading to a decrease in flow [24].

Taking the 10 d impregnation time as an example, at this point, the outlet CH₄ flow rate was 1.17 times the CO₂ flow rate. This indicates that CO₂ encountered greater resistance to migration in the coal than CH₄. The main reasons for this phenomenon are as follows:

(1)

The higher dynamic viscosity of CO₂: Under the same pressure conditions, CO₂ has a higher dynamic viscosity than CH₄, which leads to greater flow resistance along the flow path [25].

(2)

CO₂’s tendency to enter micronanopores: CO₂ is more likely to enter micronano-sized pores due to its molecular properties, and it has a higher specific surface area compared with CH₄. This allows CO₂ to adsorb onto the surface of coal, increasing local resistance.

(3)

Transition of CO₂ into a liquid-like state: As the injection pressure increases, CO₂ undergoes a physical transition toward a liquid-like state, while CH₄ remains in a gaseous state. The interactions between CO₂ and the coal matrix create more resistance than those between CH₄ and the coal. Thus, after impregnation, the permeability of CH₄ in the coal is stronger than that of CO₂.

3.4. Relationship Between Outlet Flow Rate and Inlet Pressure of Single-Component CO₂ and CH₄ Gases with Different Leaching Times

Based on the results of the CO₂ and CH₄ gas seepage experiments, the relationship curves between the outlet flow rate of the single-component CO₂ and CH₄ gas and the injection pressure were plotted, as shown in Figure 9.

In the graph, it can be seen that the outlet flow rates of the CO₂ and CH₄ gases increased with the injection pressure in different impregnation time stages. The outlet flow rate changed with the pressure in two phases: a nonlinear phase (1–5 MPa) and a linear phase (5–7 MPa). As the injection pressure increased, the nonlinear characteristics gradually became less pronounced, and the gas flow transitioned toward the linear phase. This is because the gas flow in the coal was not only influenced by viscous resistance. Additionally, during the impregnation process, supercritical CO₂ caused the extraction and dissolution of some organic and inorganic components within the coal, leading to the detachment of some organic matter from the coal’s macromolecular framework. The sample’s pores gradually increased, and the pores were connected through throats, with the pores and throats being more evenly developed. Since the binding force between small molecular substances and the coal’s macromolecular network structure was weak, it was easily weakened or destroyed and could be dissolved out by the solvent [26]. After the residual coal was extracted and dried, more transition pores and mesopores became apparent. This resulted in the transition of the gas flow from a nonlinear to a linear phase as the injection pressure increased during different impregnation times.

3.5. Relationship Between Seepage Pressure Gradient and Permeability of Single-Component CO₂ and CH₄ Gases with Different Leaching Times

The permeability of coal represents the ease with which fluids can pass through the coal column. Figure 10 illustrates the variation in the permeability of the single-component gases at different pressure gradients over different impregnation times. From Figure 10a, it follows that as the impregnation time of the coal column increased, the permeability of CO₂ gas initially increased and then decreased. Between 5 d and 10 d of impregnation, the permeability showed an increasing trend, with an increasing rate of 131.7%. However, from 15 d to 20 d, the permeability started to decline, with a decreasing rate of 18.8%. Nevertheless, compared with the raw coal sample, the CO₂ permeability still showed an overall improvement. When the coal column was impregnated for 10 days, CO₂ gas molecules could move most easily through the coal column, and the maximum CO₂ permeability was about 17.24 × 10⁻¹⁵ m². This represented a 3.2-fold increase in permeability compared with the coal column with raw coal sample impregnation, indicating that the increase in the impregnation time enhanced the permeability of the coal column.

As the pressure gradient increased, the permeability of CO₂ gas with different soaking times showed a pattern of initially decreasing and then increasing. The reason for this phenomenon is that, in the early stages of supercritical CO₂ soaking, the pore structure of the coal gradually changed. The originally closed or narrow pores may have opened or expanded, making the gas flow channels smoother and, thus, increasing the flow rate. Secondly, after 10 d of soaking, due to the leaching effect, minerals within the coal were dissolved, causing the small pores to develop and transform into medium and large pores. This resulted in the transformation of the adsorption space into flow space, which increased the permeability. After 20 d of soaking, the coal structure underwent excessive changes. For example, some pores collapsed due to over-leaching, obstructing the flow channels and causing a decrease in permeability.

Additionally, during the supercritical CO₂ dissolution process, CO₂ occupied a significant number of adsorption sites within the pores and fractures of coal. Previous experimental studies have demonstrated that the reduction in surface free energy during CO₂ adsorption is substantially greater than that during CH₄ adsorption, indicating a higher adsorption capacity of CO₂ compared with CH₄ on the coal matrix surface [27]. This process causes an adsorption-induced swelling effect, resulting in increased flow resistance and reduced seepage velocity of CO₂ within the coal seams [28]. As the pressure gradient increases, the effective stress gradually decreases, thereby enhancing CO₂ seepage through the coal formation [29]. During CO₂ adsorption, competitive adsorption occurs between CO₂ and CH₄ in the coal’s pores and fractures. Under identical conditions, gaseous CH₄ possesses a lower dynamic viscosity than CO₂, endowing it with a greater migratory capacity. Consequently, CH₄ predominantly exists in a free state within the pore-fracture system. Therefore, in the CH₄ permeability experiments, the permeability of the coal samples was higher compared with CO₂. The CH₄ permeability reached its maximum value of 18.30 × 10⁻¹⁵ m² after 10 d of soaking, as shown in Figure 10b. The maximum values for 5 d, 15 d, and 20 d were 7.31 × 10⁻¹⁵ m², 13.98 × 10⁻¹⁵ m², and 11.39 × 10⁻¹⁵ m², respectively. Compared with the permeability of the raw coal sample, which was 5.70 × 10⁻¹⁵ m², the permeability after 5 d, 10 d, 15 d, and 20 d increased by 28.24%, 145.26%, and 99.8%, respectively. As the soaking time and pressure gradient increased, the permeability of free CH₄ in the coal body gradually increased due to the changes in the pore structure and the leaching effect of supercritical CO₂. These factors worked together to cause an increase in the permeability rate of the coal body.

4. Conclusions

Based on the results of this study, the following conclusions can be drawn:

(1)

The evolution of the coal′s pore structure with the supercritical CO₂ immersion time showed a staged development characteristic. The original coal sample had a porosity of 2.06%, and after immersion for 5 d, 10 d, 15 d, and 20 d, the porosities increased by 2.78%, 3.26%, 3.22%, and 2.86%, respectively. This evolution process revealed a three-stage variation pattern of the coal porosity: “increase–stabilization–decline”. Based on this, it is suggested that in practical engineering applications, the treatment time should be controlled at around 10 d to achieve the optimal permeability enhancement effect.

(2)

After the supercritical CO₂ soaking, during the single-component gas permeability experiment, the outlet flow rates of both CO₂ and CH₄ showed an initial increase followed by a decrease as the soaking time progressed. Both gases reached their maximum outlet flow at 10 d of soaking. Compared with the raw coal sample, the outlet flow rates of CO₂ and CH₄ at 10 d increased by 4.49 times and 3.23 times, respectively. After soaking, the CH₄ permeability in the coal body was stronger than that of CO₂.

(3)

As the pressure gradient increased, the permeability of the CO₂ gas for different soaking times showed a characteristic of initially decreasing and then increasing. In contrast, the permeability of the CH₄ gas showed a nonlinear increasing trend. When the supercritical CO₂ soaking time reached 10 d, due to the extraction and leaching effects of supercritical CO₂, the adsorption space within the coal body’s pores transformed into a flow space, resulting in the best permeability for both CO₂ and CH₄ gases during this stage.

This study investigated the evolution of the coal body porosity and gas seepage characteristics of CO₂/CH₄ under the influence of the supercritical CO₂ solvent immersion time. The findings have theoretical implications for enhancing coalbed methane extraction and achieving CO₂ geological storage. However, due to limitations in research methods and experimental conditions, there are still aspects of this study that require further exploration. The next step will involve researching the pore structure evolution and seepage characteristics of coals at different ranks subjected to supercritical CO₂ immersion, based on the time effect. Additionally, this study will further explore the impact of the immersion time on the coalbed methane extraction efficiency and the micro-mechanism of CO₂ sequestration.